C3, C4, and CAM Plants Overview

Study Notes

C3, C4, and CAM plants are different types of photosynthetic pathways
C4 plants use the Hatch-Slack pathway which occurs first before the Calvin cycle
C4 plants have PEP carboxylase, which adds CO2 to PEP, a 3-carbon compound, in the mesophyll cells producing a 4-carbon compound.
This 4-carbon molecule moves into the bundle-sheath cells via plasmodesmata
Inside the bundle sheath cells, the CO2 is released and the Calvin cycle begins.
The process minimizes photorespiration
C4 plants can maintain a high concentration of CO2 in bundle-sheath cells, thus avoiding wasteful photorespiration
CAM (Crassulacean Acid Metabolism) plants are adapted to arid conditions
Stomata in CAM plants are closed during the day and open at night to take up CO2
CAM plants convert CO2 into 4-carbon acids during the night and then release the CO2 during the day for use in the Calvin cycle
CAM plants fix carbon into 4-carbon acids using PEP carboxylase and storing it.
CAM and C4 plants use similar overall pathways but differ in when carbon dioxide is taken up.
C3 plants lose about one-third of their photosynthetic output to photorespiration while it is much lower in C4 and CAM plants.

C4 plants are able to maintain a high concentration of CO2 in the bundle-sheath cells, avoiding photorespiration
C3 plants have photorespiration
C4 plants have a higher optimal temperature for photosynthesis (30-45°C) than C3 plants (20-25°C)
C4 plants have a higher rate of photosynthesis at high light intensity
C3 plants show a declining quantum yield of photosynthesis as temperature increases, while C4 plants are relatively steady.
C3 plants use Rubisco, while C4 plants use PEPcase

C4 species have chloroplasts in the vascular sheath cells. C3 species do not.
Chloroplasts in mesophyll cells of C3 and C4 plants are structurally similar.
Starch is not produced in C4 plants (only 4C compounds)
Chloroplasts in the vascular sheath cells of C4 plants are larger and have less-developed grana than those in the mesophyll cells.
Calvin cycle occurs in C4 plants in bundle sheath cells, while Calvin cycle and other processes happen in the same cell in CAM plants.

The first product when CO2 is fixed is the same (a 4-carbon molecule)
Different time of the day to fix CO2 (CAM at night, C4 during daytime)
Fixation of CO2 and Calvin Cycle occur in different cells in C4. However, in CAM they both occur in the same cell.

C4 plants minimize photorespiration by incorporating carbon dioxide into 4-carbon compounds in the mesophyll cells (e.g., phosphoenolpyruvate, pyruvate, malate, oxaloacetate).
These four-carbon compounds are exported to the bundle sheath cells where they release CO2 for use in the Calvin Cycle
During the day stomata close and the CO2 released from the organic acids used in the Calvin Cycle

CAM plants open their stomata at night to take up CO2 and incorporate it into organic acids.
CO2 is stored at night. Then used during the day.

C3: Mesophyll with no distinct bundle sheath. Rubisco as carboxylating enzyme. High photorespiration.
C4: Mesophyll with a distinct bundle sheath. PEPcase and Rubisco (separation in space) as carboxylating enzymes. Very low photorespiration.
CAM: Mesophyll with a distinct bundle sheath. PEPcase and Rubisco (separation in time) as carboxylating enzymes. Very low photorespiration.